The SWIG tool helps developers in building interfaces/libraries that can be accessed from many other languages than the ones the library is initially written in or for. The SELinux userland utility setools uses it to provide Python and Ruby interfaces even though the application itself is written in C. Sadly, the tool currently requires swig-1 for its building of the interfaces and uses constructs that do not seem to be compatible with swig-2 (same with the apse package, btw).
I first tried to patch setools to support swig-2, but eventually found regressions in the libapol library it provides so the patch didn’t work out (that is why some users mentioned that a previous setools version did build with swig – yes it did, but the result wasn’t correct). Recently, a post on Google Plus’ SELinux community showed me that I wasn’t wrong in this matter (it really does require swig-1 and doesn’t seem to be trivial to fix).
Hence, I have to fix the gentoo build problem where one set of tools requires swig-1 and another swig-2. Otherwise world-updates and even building stages for SELinux systems would fail as Portage finds incompatible dependencies. One way to approach this is to use Gentoo’s support for SLOTs. When a package (ebuild) in Gentoo defines a SLOT, it tells the package manager that the same package but a different version might be installed alongside the package if that has a different SLOT version. In case of swig, the idea is to give swig-1 a different slot than swig-2 (which uses SLOT="0") and make sure that both do not install the same files (otherwise you get file collisions).
Luckily, swig places all of its files except for the swig binary itself in /usr/share/swig/<version>, so all I had left to do was to make sure the binary itself is renamed. I chose to use swig1.3 (similar as to how tools like ruby and python and for some packages even java is implemented on Gentoo). The result (through bug 466650) is now in the tree, as well as an adapted setools package that uses the new swig SLOT.
Thanks to Samuli Suominen for getting me on the (hopefully ;-) right track. I don’t know why I was afraid of doing this, it was much less complex than I thought (now let’s hope I didn’t break other things ;-)